Sweeping is a purification process that removes certain impurities and thereby improves the radiation and etching properties of quartz crystals. It is an electric-field driven, high-temperature, solid-state diffusion process. The major steps of a typical sweeping process include: applying electrodes to the Z-surfaces of a lumbered quartz bar; applying voltage to the electrodes to produce an electric field of approximately 1 KV/cm; heating the bar slowly in either air, vacuum, or in hydrogen-containing or inert atmospheres to about 500.degree. C; monitoring the current through the bar (as sweeping progresses, the current decreases); when the current decays to some near-constant value, cooling the bar slowly to room temperature, and removing the voltage.
Alkali metal impurities, such as Li.sup.+ and Na.sup.+ are charge compensators for trivalent aluminum impurities in quartz. When sweeping is performed in air or H.sub.2, the alkalis diffuse to the cathode and are replaced by the indiffusion of hydrogen at the anode. The alkali-metal impurities are subsequently removed by lapping the Z-surfaces. The sweeping process also modifies dislocations, reducing the tendency for the formation of etch channels. Thus, sweeping can provide a substantial improvement in quartz resonator etch-processibility.
Heretofore, either evaporated-metal or metal foil electrodes have been used for sweeping quartz material as for example, electrodes for quartz resonator devices and interdigital transducer (IDT) fingers in surface acoustic wave (SAW) devices.
The difficulty with evaporated-metal or metal foil electrodes is that they are subject to suppression of hydrogen diffusion and nonuniformities derived from discontinuous areal contact of the electrode with the quartz.